Critical dimension analysis with simultaneous multiple angle of incidence measurements

ABSTRACT

A method and apparatus are disclosed for evaluating relatively small periodic structures formed on semiconductor samples. In this approach, a light source generates a probe beam which is directed to the sample. In one preferred embodiment, an incoherent light source is used. A lens is used to focus the probe beam on the sample in a manner so that rays within the probe beam create a spread of angles of incidence. The size of the probe beam spot on the sample is larger than the spacing between the features of the periodic structure so some of the light is scattered from the structure. A detector is provided for monitoring the reflected and scattered light. The detector includes multiple detector elements arranged so that multiple output signals are generated simultaneously and correspond to multiple angles of incidence. The output signals are supplied to a processor which analyzes the signals according to a scattering model which permits evaluation of the geometry of the periodic structure. In one embodiment, the sample is scanned with respect to the probe beam and output signals are generated as a function of position of the probe beam spot.

PRIORITY

[0001] This application is a continuation of U.S. application Ser. No.10/150,032, filed May 17, 2002, which is in turn a continuation of U.S.application Ser. No. 09/818,703, filed Mar. 27, 2001, now U.S. Pat. No.6,429,942, which claimed priority to provisional Application Serial No.60/192,899, filed Mar. 29, 2000.

TECHNICAL FIELD

[0002] The subject invention relates to optical metrology equipment formeasuring critical dimensions and feature profiles of periodicstructures on semiconductor wafers. The invention is implemented usingdata obtained from simultaneous multiple angle of incidence measurementsas an input to analytical software designed to evaluate surface featuresvia a specular scatterometry approach.

BACKGROUND OF THE INVENTION

[0003] There is considerable interest in the semiconductor industry inevaluating small features of periodic structures on the surface of asample. In current high density semiconductor chips, line widths orfeature sizes are as small as 0.1 microns. These feature sizes are toosmall to be measured directly with conventional optical approaches. Thisis so because the line widths are smaller than the probe beam spot sizewhich can be achieved with most focusing systems.

[0004] This problem is illustrated in FIG. 1 which shows a wafer 10having formed thereon a number of conductive lines 12. A probe beam 14is shown focused by a lens 20 onto the sample at a spot 16. Thereflected beam is measured by a photodetector 18. As can be seen, spot16 overlaps multiple lines 12 and therefore cannot be used to measuredistances between lines or the thickness of the lines themselves.

[0005] To overcome this problem, sophisticated software programs havebeen developed which analyze the reflected probe beam in terms of ascattering model. More specifically, it is understood that criticaldimensions or feature profiles on the surface of the wafer will causesome level of scattering of the reflected probe beam light. If thisscattering pattern is analyzed, information about the criticaldimensions can be derived. This approach has been called specularscatterometry. The algorithms use various forms of modeling approachesincluding treating the lines as an optical grating. These algorithmsattempt to determine the geometry of the periodic structure.

[0006]FIG. 2 schematically illustrates the geometry of one type ofperiodic structure 24. This periodic structure can be analyzed in termsof the width W between the features and the depth D of the grooves. Inaddition, the shape or profile P of the side walls of the features canalso be analyzed by the current algorithms operating on the analyticaldata.

[0007] To date, these analytical programs have been used with data takenfrom conventional spectroscopic reflectometry or spectroscopicellipsometry devices. In addition, some efforts have been made to extendthis approach to analyzing data from simultaneous multiple angle ofincidence systems. In these systems, the spot size is relatively small,but still larger than the individual features of the periodic structure.Paradoxically, where the features are only slightly smaller than thespot size, analysis through scatterometry is difficult since not enoughof the repeating structure is covered by the spot. Accordingly, it wouldbe desirable to modify the system so a sufficient number of individualfeatures are measured so a good statistically based, scatterometryanalysis can be performed.

SUMMARY OF THE INVENTION

[0008] The assignee of the subject invention has previously developedsimultaneous multiple angle of incidence measurement tools which havebeen used to derive characteristics of thin films on semiconductorwafers. It is believed that data from the same type of tools can be usedwith an appropriate scattering model analysis to determine criticaldimensions and feature profiles on semiconductors.

[0009] Detailed descriptions of assignee's simultaneous multiple angleof incidence devices can be found in the following U.S. Pat. Nos.4,999,014; 5,042,951; 5,181,080; 5,412,473 and 5,596,411, allincorporated herein by reference. The assignee manufactures a commercialdevice, the Opti-Probe which takes advantage of some of thesesimultaneous, multiple angle of incidence systems. A summary of all ofthe metrology devices found in the Opti-Probe can be found in PCTapplication WO/9902970, published Jan. 21, 1999.

[0010] One of these simultaneous multiple angle of incidence tools ismarketed by the assignee under the name beam profile reflectometer(BPR). In this tool, a probe beam is focused with a strong lens so thatthe rays within the probe beam strike the sample at multiple angles ofincidence. The reflected beam is directed to an array photodetector. Theintensity of the reflected beam as a function of radial position withinthe beam is measured and includes not only the specularly reflectedlight but also the light that has been scattered into that detectionangle from all of the incident angles as well. Thus, the radialpositions of the rays in the beam illuminating the detector correspondto different angles of incidence on the sample plus the integratedscattering from all of the angles of incidence contained in the incidentbeam. In this manner, simultaneous multiple angle of incidencereflectometry can be performed.

[0011] Another tool used by the assignee is known as beam profileellipsometry. In one embodiment as shown and described in U.S. Pat. No.5,042,951, the arrangement is similar to that described for BPR exceptthat additional polarizers and/or analyzers are provided. In thisarrangement, the change in polarization state of the various rays withinthe probe beam are monitored as a function of angle of incidence.

[0012] It is believed that the data generated by either of these toolscould be used to appropriately model and analyze critical dimensions andfeature profiles on semiconductors.

[0013] The lens used to create the probe beam spot from a laser sourcein the above two simultaneous multiple angle of incidence systems istypically larger than the distance between adjacent features of theperiodic structure of interest. However, in order to providestatistically significant information, it is desirable that informationbe collected from at least twenty or more of the repeating features. Onemethod of achieving this goal is to increase the spot size of the probebeam. Such an approach is described in U.S. Pat. No. 5,889,593incorporated by reference. In this patent, a proposal is made to includean optical imaging array for breaking up the coherent light bundles tocreate a larger spot.

[0014] It is believed the latter approach is not desirable because ofthe additional complexity it introduces into the measurement. Ideally,when attempting to analyze a periodic structure (e.g., a periodiccritical dimension array) it is desirable to have no additionalperiodicities in the measurement system between the source and detector.Multiple periodic signals are more difficult to analyze and are oftenplagued with added uncertainty and ambiguity with respect to extractingparameters associated with any of the constituent components.

[0015] In accordance with the subject invention, the requirement forincreasing the area over which measurements are taken is achieved in twodifferent ways. In the first approach, the probe beam spot is scannedover the wafer until a sufficient amount of data are taken. Once thedata are taken, a spatial averaging algorithm is utilized. Spatialaveraging is discussed in U.S. patent application Ser. No. 09/658,812,filed Sep. 11, 2000 and incorporated herein by reference.

[0016] In another approach, the probe beam is generated by an incoherentor white light source. When incoherent light is focused by a lens, thespot size will be significantly larger than with a laser. No separateimaging array needs to be included to break up the coherence of thelight as in the prior art. In such a system, a monochrometer could belocated between the light source and the detector to permit measurementof a narrow band of wavelengths. The wavelength selected can be matchedto the type of sample being inspected in order to obtain the moststatistically relevant data. In addition, it would also be possible toscan the monochrometer in order to capture data at multiple wavelengths.It would also be possible to measure multiple wavelengths simultaneouslyas described in U.S. Pat. No. 5,412,473.

[0017] Alternatively or in addition, the measurement data which can beobtained from two or more metrology devices of the type described in theabove identified PCT application, could be used to advance thisanalysis. As more of these metrology devices are added, the ability tounambiguously distinguish features increases. Thus, it is within thescope of the subject invention to utilize either or both of asimultaneous multiple angle of incidence spectrometer or ellipsometeralong with one or more of spectroscopic reflectometry, spectroscopicellipsometry or absolute ellipsometry tools with the latter two beingdeployed in a manner that maximizes the information content of themeasurement. For example, with a rotating compensator spectroscopicellipsometer one measures both the sign and magnitude of theellipsometric phase while in more standard configurations, e.g., arotating polarizer/rotating analyzer, only the magnitude or phase can bemeasured.

[0018] An example of an analytical approach for evaluating criticaldimensions using data from a broadband reflectometer is described in“In-situ Metrology for Deep Ultraviolet Lithography Process Control,”Jakatdar et. al. SPIE Vol. 3332, pp. 262-270 1998. An example of using aspectroscopic ellipsometer equipment for CD metrology is described in,“Specular Spectroscopic Scatterometry in DUV Lithography, SPIE Vol.3677, pp 159-168, from the SPIE Conference on Metrology, Inspection andProcess Control for Microlithography XIII, Santa Clara, Calif., March1999.

[0019] Further and related information measuring critical dimensions canbe found in U.S. Pat. Nos. 5,830,611 and 5,867,276, incorporated hereinby reference.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is schematic diagram illustrating the optical measurementof periodic structure on a sample.

[0021]FIG. 2 is a cross-sectional illustration of the type of periodicstructure which can be measured in accordance with the subjectinvention.

[0022]FIG. 3 is a schematic diagram of an apparatus for performing themethod of the subject invention.

[0023]FIG. 4 is a schematic diagram illustrating an alternate embodimentof the subject apparatus for performing spectroscopic measurements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Turning to FIG. 3, a basic schematic of simultaneous multipleangle of incidence apparatus 30 is illustrated. Further details aboutsuch a device are described in U.S. Pat. Nos. 4,999,014; 5,042,951;5,159,412 and 5,412,473 all incorporated herein by reference. As notedabove, the assignee's Opti-Probe device incorporates portions of thistechnology and markets the measurement subsystem under the trademarkBeam Profile Reflectometry or BPR. In the past, the BPR technology wasutilized primarily to analyze the characteristics of thin films formedon semiconductors. This disclosure is directed to using the measurementswhich can be obtained from this type of system to evaluate the geometryof periodic structures formed on semiconductors.

[0025] The basic measurement system includes a light source 32 forgenerating a probe beam 34. The light source can be a laser forgenerating a coherent beam of radiation. Laser diodes are suitable lasersources for this application. If the output of the laser is not itselfpolarized, a separate linear polarizer can be provided. As discussedbelow, light source 32 can also be a polychromatic or white light sourcefor generating a probe beam with a plurality of wavelengths.

[0026] The probe beam 34 is focused onto the sample 10 using a lens 40in a manner so that the rays within the probe beam create a spread ofangles of incidence. In the preferred embodiment, the beam is directednormal to the surface but can be arranged off-axis as illustrated inU.S. Pat. No. 5,166,752, incorporated by reference. Lens 40 ispreferably a high numerical aperture lens (on the order of 0.90) tocreate angles of incidence from zero to about 70 degrees. The lenscreates rays having predominantly S-polarized light along one axis andpredominantly P-polarized light along an orthogonal axis. Atintermediate angles, the polarization is mixed.

[0027] Lens 40 is positioned to create a probe beam spot 42 on thesample on the order of about 1 micron in diameter where the light sourceis coherent (i.e. a laser source). This spot is typically somewhatlarger than the spacing (width W) between the recurring features of theperiodic structure. For this reason, a certain portion of the light fromthe probe beam will be diffracted or scattered from the periodicstructure. As discussed below, this light can be analyzed with ascattering model in a manner similar to prior art probe beam detectionscatterometry systems. The advantage of the subject approach is that thedata can be simultaneously obtained from a plurality of angles ofincidence.

[0028] In order to obtain data sufficient to perform an accurateevaluation, it is preferable that the probe beam collect informationfrom at least 20 repeating features in the pattern. If the probe beamspot 42 is not sufficiently large, than it would be desirable to scanthe probe beam over the surface of the sample in the region of theperiodic structure. This can be accomplished by moving an X-Y stage 44upon which the sample rests. It would also be possible to providescanning capability to the probe beam itself. Scanning would preferablybe in a direction perpendicular to the parallel features of the periodicstructure. Data is generated as a function of the position of the probebeam spot with respect to the features of the periodic structure. Wherethe sample is scanned, the data is analyzed as discussed above andfurther clarified using a spatial averaging algorithm. In this spatialaveraging approach, the data from points in the scan are filtered by arepeated sequence of averaging and outlier exclusions where the outliersare defined by their differences with respect to signal levels andsymmetry properties. The result of this process leads to data that areequivalent to those taken with an incoherent source illuminating an areathe same as that scanned in the spatial averaging approach.

[0029] The reflected/scattered beam passes back up through the lens 40which collimates the beam. The reflected beam is redirected by asplitter to an imaging lens 48. Lens 48 magnifies and relays an image ofthe sample at the focal plane of the lens. A spatial filter 50 having anaperture is placed in the focal plane of the lens 48 for controllingsize of the area of the sample which is measured.

[0030] The probe beam is than passed through a 50-50 splitter anddirected to two photodetectors 54 and 56 having a linear array ofdetector elements. The photodetectors are arranged orthogonal to eachother to measure both the S and P polarization components. As describedin detail in the above cited patents, each of the detecting elements inthe array measure different angles of incidence. The radial positionwithin the reflected probe beam is mapped to the angle of incidence,with the rays closer to the center of the beam having the smallestangles of incidence and the rays in the radially outer portion of thebeam corresponding to the greatest angles of incidence. Thus, eachdetector element simultaneously generates an independent signals thatcorrespond to a different angle of incidence.

[0031] The output signals from the detector arrays are supplied to theprocessor 60. Processor will analyze the signals based on algorithmwhich considers the reflected and scattered light such as a rigorouscoupled wave analysis. The selected algorithm will correlate thevariation in reflectivity as a function of angle of incidence with thegeometry of the periodic structure. Such scattered light theoreticalmodels are well known in the literature. In addition to the articlescited above, further examples can be found in the following articleswhich are cited by way of example. Those skilled in the art of analyzingsignals diffracted from periodic structures will understand that thereare many other approaches which can be utilized. It should be noted thatsince this approach obtains measurements at multiple angles ofincidence, higher order diffraction effects may be collected andconsidered.

[0032] Prior Articles:

[0033] 1. “Optical Etch-Rate Monitoring: Computer Simulation ofReflectance,” Heimann and Schultz, J. Electrochem. Soc: Solid StateScience and Technology, April 1984, Vol. 131, No. 4, page 881

[0034] 2. “Optical Etch-Rate Monitoring Using Active Device Areas:Lateral Interference Effects”, Heimann, J. Electrochem. Soc: Solid StateScience and Technology, August 1985, Vol. 132, No. 8, page 2003.

[0035] 3. “Scatterometry for 0.24 micron-0.70 micron developedphotoresist metrology,” Murnane et. al. SPIE, Vol. 2439, page 427(1995).

[0036] 4. “Multi-Parameter Process metrology using scatterometry,”Raymond et. al. SPIE Vol. 2638, page 84 (1995).

[0037] 5. “Specular Spectral Profilometry on Metal Layers,” Bao et. al,SPIE Vol 3998 (2000), page 882.

[0038] The type of analysis will depend on the application. For example,when used for process control, either in situ or near real time, theprocessor can compare the detected signals to an expected set of signalscorresponding to the desired geometry of the periodic structure. If thedetected signals do not match the expected signals, it is an indicationthat the process is not falling within the specified tolerances andshould be terminated and investigated. In this approach, nosophisticated real time analysis of the signals is necessary

[0039] As is known in the art, the reflected output signals at multipleangles of incidence can be more rigorously analyzed to determine thespecific geometry of the periodic structure. While there are a number ofdifferent approaches, most have certain traits in common. Morespecifically, the analytical approach will typically start with atheoretical “best guess” of the geometry of the measured structure.Using Fresnel equations covering both the reflection and scattering oflight, calculations are made to determine what the expected measuredoutput signals would be at different angles of incidence for thetheoretical geometry. These theoretical output signals are compared tothe actual measured output signals and the differences noted. Based onthe differences, the processor will generate a new set of theoreticaloutput signals corresponding to a different theoretical periodicstructure. Another comparison is made to determine if the theoreticalsignals are closer to the actual measured signals. These generation andcomparison steps are repeated until the differences between thetheoretically generated data and the actually measured data aresubstantially minimized. Once the differences have been minimized, thetheoretical periodic structure corresponding to the best fit theoreticaldata is assumed to represent the actual periodic structure.

[0040] This minimization procedure can be carried out with aconventional least squares fitting routine such as a Levenberg-Marquardtalgorithm. It would also be possible to use a genetic algorithm. (See,U.S. Pat. No. 5,953,446.)

[0041] In the past, this type of rigorous analysis was limited to theresearch environment, since the calculations necessary to determine theperiodic structure was extremely complex and time consuming. However,with advent of faster and parallel processing technologies, it isbelieved that such an analytical approach could be used in a real timeanalysis.

[0042] One method for reducing the computer processing time duringmeasurement activities is to create a library of possible solutions inadvance. (See the Jakatdar articles, cited above). In this approach, arange of possible periodic structures and their associated theoreticaloutput signals are generated in advance using the Fresnel equations asdiscussed above. The results are stored as a library in a processormemory. During the measurement activities, the actual measured signalsare compared with sets of theoretically generated output signals storedin the library. The periodic structure associated with the set oftheoretical signals which most closely matches the actual measured datais assumed to most closely represent the geometry of the measuredperiodic structure.

[0043] The simultaneous multiple angle approach is not limited toreflectometry. As noted in U.S. Pat. Nos. 5,042,951 and 5,166,752(incorporated herein by reference), it is also possible to obtainellipsometric measurements corresponding to ψ and Δ simultaneously atmultiple angles of incidence. To obtain such measurements, someadditional optical elements should be added to the device of FIG. 3. Forexample, a polarizer 66 (shown in phantom) is desirable to accuratelypredetermine the polarization state of the probe beam. On the detectionside, an analyzer 68 (also shown in phantom) is provided to aid inanalyzing the change in polarization state of the probe beam due tointeraction with the sample. The optical components of the analyzer canbe of any type typically used in an ellipsometer such as a polarizer ora retarder. The ellipsometric output signals are analyzed in a fashionsimilar to the prior art approaches for using ellipsometric data toevaluate the geometry of periodic structures.

[0044] Another approach to increasing the size of the probe beam spot isto use an incoherent source for the probe beam. Such an incoherentsource can include a variety of well known spectral line or broad bandsources. If a spectral line light source is used, some modest level ofnarrow pass filtering may be desirable. Such a filter could be locatedeither before the sample or before the detector as indicated in phantomlines 69 a and 69 b. The wavelength which is used is selected in orderto maximize the sensitivity in the reflection response to the type ofchanges of interest.

[0045] It would also be possible to use a broadband or white lightsource generating a polychromatic beam. In this situation, thewavelength selective filter could be in the form of a conventionalmonochrometer. A monochrometer, which typically includes a dispersiveelement and a slit, functions to transmit a narrow band of wavelengths.The system could be arranged to take measurements at only one wavelengthor in a series of sequential wavelengths as the monochrometer is tuned.The use of an incoherent light source would fill the field of view onthe sample (typically 100 microns or more for a 0.9 NA microscopeobjective). The actual measurement spot size is controlled by anaperture that can be varied in size as needed for the particularmeasurement in question. Such variable spatial filtering is described inU.S. Pat. No. 5,412,473.

[0046] It would also be possible to set up a system where both themultiple angle and multiple wavelength information is obtainedsimultaneously. Such a detection scheme is also described in detail inU.S. Pat. No. 5,412,473, incorporated by reference. This detectionscheme is briefly described herein with reference to FIG. 4.

[0047] In this embodiment, the probe beam 34 a is a broadbandpolychromatic beam generated by a white light source 32 a generating anincoherent probe beam. There are a number of white light sourcesavailable such as tungsten or deuterium bulbs. The probe beam 34 a isfocused on the sample with lens 40. Upon reflection, the probe beam ispassed through relay lens 48 and spatial filter 50 in the mannerdescribed above. In addition, the beam is passed through a filter 70having an slit 72 located in the relay image plane of the exit pupil oflens 40. Lens 48 also serves to relay this image. The slit isdimensioned so that image transmitted to the detector 74 will be on theorder of the dimensions of a row of detector elements 76.

[0048] After the beam passes through the slit, it is dispersed as afunction of wavelength by element 80. Any conventional wavelengthdispersing element can be used, such as a grating, prism or holographicplate.

[0049] The dispersed beam is directed to the detector which is a twodimensional array of photodiodes. A CCD element could also be used. Theslit 72 is oriented perpendicular to the axis of the dispersion of thelight. In this matter, each horizontal row of elements on the array 74will measure a narrow wavelength band of light. Each of the elements ineach row correspond to different angles of incidence. Thus, the outputof the detector 74 will simultaneously produce data for multiplewavelengths and multiple angles of incidence. As noted in U.S. Pat. No.5,412,473, this type of detection system can be used for eitherreflectometry or ellipsometry measurements.

[0050] It is also within the scope of the subject invention to combinethese measurements with other measurements that might be available froma composite tool. As noted above, the assignee's Opti-Probe device (asdescribed in WO 99/02970) has multiple measurement technologies inaddition to the Beam Profile Reflectometry system. These othertechnologies include broadband reflectometry and broadband ellipsometry.The output from these additional modules can be used in combination withthe BPR signals to more accurately evaluate the geometry of the periodicstructures.

[0051] In summary, there has been described a method and apparatus forevaluating relatively small periodic structures formed on semiconductorsamples. In this approach, a light source generates a probe beam whichis directed to the sample. In one embodiment, the light source generatesincoherent light. A lens is used to focus the probe beam on the samplein a manner so that rays within the probe beam create a spread of anglesof incidence. The size of the probe beam spot on the sample is largerthan the spacing between the features of the periodic structure so someof the light is scattered from the structure. A detector is provided formonitoring the reflected and scattered light. The detector includesmultiple detector elements arranged so that multiple output signals aregenerated simultaneously and correspond to multiple angles of incidence.The output signals are supplied to a processor which analyzes thesignals according to a scattering model which permits evaluation of thegeometry of the periodic structure. Both single and multiple wavelengthembodiments are disclosed.

[0052] While the subject invention has been described with reference toa preferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

We claim:
 1. A method for monitoring the processing of semiconductorwafers, wherein the processing creates one or more geometrical featureson the surface of the wafer having at least one dimension significantlyless than a micron, said method comprising the steps of: focusing acoherent probe beam of radiation to a spot overlapping the feature onthe sample surface in a manner so that the rays within the probe beamcreate a spread of angles of incidence and wherein the spot size on thesample is on the order of one micron in diameter so that the probe beamis diffracted upon reflection; monitoring the diffracted probe beamlight and simultaneously generating a plurality of independent outputsignals corresponding to a plurality of different angles of incidence;and comparing the output signals to an expected set of signals todetermine if the process is within a specified tolerance.
 2. A method asrecited in claim 1, wherein said process is terminated if the outputsignals are outside said specified tolerance.
 3. A method as recited inclaim 1, wherein said probe beam is focused onto the sample while thesample is in situ.
 4. A method as recited in claim 1, wherein the probebeam is generated by a laser.
 5. A method as recited in claim 1, whereinthe probe beam is passed through an analyzer and wherein the change inpolarization state of the rays within the probe beam are monitored.
 6. Amethod as recited in claim 1, wherein the expected set of signals aregenerated based on a theoretical profile of the feature and whereinafter the comparison step, another set of expected signals are generatedbased on the results of the comparison using a different theoreticalprofile of the feature and wherein the comparison and generation stepsare repeated until the differences between the expected set of signalsand the output signals are minimized.
 7. A method as recited in claim 1,wherein the during the comparison step, the output signals are comparedto a set of previously generated theoretical expected signals to findthe closest match and wherein each one of the set of previouslygenerated expected signals corresponds to a different possible geometryof the feature.